The New York Times ran a very interesting story about how we have engineered much of the nutrition out of our food. Some quotes:

Wild dandelions, once a springtime treat for Native Americans, have seven times more phytonutrients than spinach, which we consider a “superfood.” 
A purple potato native to Peru has 28 times more cancer-fighting anthocyanins than common russet potatoes.
One species of apple has a staggering 100 times more phytonutrients than the Golden Delicious

So how did we get here? Well, its started when we truly started farming (versus foraging). Farmers over the generations have chosen plants that are higher in sugar, oil, and starch as well as lower in fiber. It turns out that these characteristics are inversely related to the amount of phytonutrients. It seems that sweeter and higher calorie food choices was around long before Nabisco and Kraft (and others) got in the game with their particularly horrendous version. 

The articles author, Jo Robinson, goes on to use corn as an example of todays state of food. The article makes some food choice recommendations, given whats available today, as well as suggest that we need to reconsider our priorities as a society. Another quote: 

The United States Department of Agriculture exerts far more effort developing disease-resistant fruits and vegetables than creating new varieties to enhance the disease resistance of consumers. In fact, I’ve interviewed U.S.D.A. plant breeders who have spent a decade or more developing a new variety of pear or carrot without once measuring its nutritional content.

An excellent read. If you want to dig even deeper, Jo Robinson published a book on the topic   Eating on the Wild Side: The Missing Link to Optimum Health